1. Field of the Invention
The present invention relates to a phase plate and also to a method of fabricating it. Further, the invention relates to an electron microscope.
2. Description of the Related Art
A sample to be examined by a transmission electron microscope (TEM) or scanning transmission electron microscope (STEM) equipped with a phase plate is called a weak phase object which, when an electron beam hits it, produces only phase variations without varying the intensity of the electron beam transmitted through the sample. In particular, examples of such a sample include light element samples such as undyed biological samples (samples embedded in resin and samples embedded in ice) and macromolecules which produce only low contrast when observed in absorption contrast or in scattering contrast.
Phase plates are employed in phase contrast transmission electron microscopes and phase contrast scanning transmission electron microscopes. Each phase plate has a thin film that is centrally provided with a quite small through-hole (e.g., having a diameter on the order of 1 micrometer). In a phase contrast transmission electron microscope, some of electron waves transmitted through a sample are varied in phase by placing a phase plate in the back focal plane of an objective lens. As a result, phase contrast can be obtained. On the other hand, in a phase contrast scanning transmission electron microscope, a phase plate is placed in the front focal plane of an objective lens. Phase contrast can be detected by irradiating a sample with electron waves of different phases.
Such a phase plate installed in a transmission electron microscope or a scanning transmission electron microscope presents the problem that degradation occurs due to electrification. For example, in patent document 1, there is disclosed a phase plate structure for preventing lens effects due to electrification because it is considered difficult to prevent electrification of the phase plate.